The U.N. monitors, whose number gradually rose to about 300, were sent in to ensure that both President Bashar al-Assad and opposition fighters were abiding by a six-point peace plan brokered by special envoy Kofi Annan.

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Violence has soared in recent days with reports of heavy government bombardment of town and chilling massacres of civilians.

On some occasions, the monitors themselves have come under fire.

"The lack of willingness by the parties to seek a peaceful transition, and the push towards advancing military positions is increasing the losses on both sides: innocent civilians, men women and children are being killed every day," Mood said.

He said U.N. observers will no longer be conducting patrols and will stay in their locations until further notice.

This suspension will be reviewed on a daily basis, Mood said.

Operations will resume when we see the situation fit for us to carry out our mandated activities.

Syrian opposition groups estimate that between 12,000 and 14,000 people have died in the months of uprising against al-Assad's rule.